"Iran tries to protect its nuclear scientists very well but sometimes things get out of hand, and incidents like this happen."

Baqer Moin, an Iranian author and journalist in London, said Iranian media have accused "Zionist agents" as being behind the blast.

"They are looking towards people who are interested in delaying the Iranian nuclear programme. Websites close to the [Iranian] authorities are making these statements," he told Al Jazeera.

"There have been in the past reports that many nuclear scientists or people who wanted to join the Iranian nuclear organisation have been intimidated.

"There also were claims of other assassinations. But this is the first time in recent months that in Tehran such an occurrence has happened."

Western powers, including the US, accuse Iran of covertly seeking to develop atomic weapons.

They demand that Iran accept a UN brokered offer that would delay Iran's ability to make a nuclear weapon as well as engage in broader talks with the ultimate goal of persuading it to stop its enrichment programme.